1. Field of the Invention
The present invention relates to a double-side image forming apparatus capable of forming images on both faces of a sheet-shaped image bearing member.
2. Description of the Prior Art
In conventional image forming apparatus such as copiers, the reproduced image is generally formed only on one face of the sheet-shaped image bearing member or copy sheet. However such copy sheets, when stored in bulk, require twice space and weight in comparison with double-faced documents such as books. Double-side image forming apparatus are required also in consideration of the economy in resources, labor power and cost.
In the following description the present invention will be explained by a double-side copier as an example of such apparatus. In a copier employing image transfer process, double-sided copying can be achieved in the following systems.
In a first process, different images are formed respectively on two photosensitive drums for top face and bottom face, and transferred to both faces of the copy sheet.
This system, though simple in principle, is disadvantageous in requiring a large expensive apparatus because of the necessity for two independent copying processes.
In a second system a first image formed on a photosensitive drum is transferred to an intermediate drum for temporary storage, then a second image is again formed on the photosensitive drum, and said images are transferred respectively from said intermediate drum and photosensitive drum to the copy sheet. This system requires only one copying process but is disadvantageous in different image quality between the top face and the bottom face.
In a third system the ordinary one-side copying is made on a face of the copy sheet, and the completed copy sheet is inverted and returned to the original paper feed section or another paper feed section for copying again on the bottom face.
In contrast to the first or second system associated with the drawback in cost and dimension of the apparatus or in the image quality, the third system is considered most practical for realizing a simple and compact double-side copier, requiring only one copying process same as in the one-side copying.
However the third system is often confronted by other troubles such as paper jamming because each copy sheet has to go through a longer path in comparison with the ordinary one-side copying and also because each copy sheet of which surface conditions have been modified by the steps of image transfer and fixing has to be again subjected to the steps of paper feeding, image transfer and fixing for the second time.